The present invention relates to signal processing, and, in particular, to techniques for tuning amplifiers that employ feed-forward compensation.
Amplifiers, such as high-power amplifiers used in the base stations of wireless communication systems, typically exhibit non-linearity over their operating ranges. This non-linearity can result in noise that can corrupt or otherwise interfere with the communications. To address this problem, additional circuitry may be added to an amplifier in an attempt to linearize the effective amplifier response. Conventional techniques for linearizing amplifiers typically involve pre-compensation and/or feed-forward compensation.
In amplifier linearization based on pre-compensation, the input signal that is to be amplified is pre-distorted prior to being applied to the amplifier in order to adjust the input signal based on known non-linearities in the amplifier transfer function. In feed-forward compensation, an error signal is fed forward and combined with the output of the amplifier to adjust the output signal for non-linearities in the amplifier transfer function.
FIG. 1 shows a high-level block diagram of a linearized amplifier circuit 100 according to the prior art. Amplifier circuit 100 utilizes feed-forward compensation to linearize the response of a high-power amplifier (HPA) 108. Amplifier circuit 100 has a main amplifying chain and an error amplifier chain. The main amplifying chain includes adjuster 102, HPA 108, tap 110, delay module 112, and coupler 114, while the error amplifier chain includes delay module 122, coupler 124, adjuster 130, and error amplifier (EA) 132. In addition, amplifier circuit 100 includes splitter 120, pilot generator 104, coupler 106, taps 116 and 126, and detectors 118 and 128. Depending on the application, adjusters 102 and 130 may typically be implemented using vector modulators.
In operation, an input signal is split at splitter 120 and applied to both adjuster 102 and delay module 122. In the main amplifying chain, the amplitude and/or phase of the signal from splitter 120 are (optionally) adjusted prior to being applied to HPA 108. If pilot generator 104 is activated, then a pilot signal is injected into the signal at coupler 106 prior to being applied to HPA 108. A portion of the amplified signal generated by HPA 108 is tapped off at tap 110 and the rest is delayed at delay module 112 (to compensate for the timing of the corresponding portion of the error amplifier chain). A feed-forward error-compensation signal (described below) from EA 132 is added to the delayed, amplified signal from delay module 112 at coupler 114 and the resulting compensated signal is provided as the output signal from amplifier circuit 100. Detector 118 monitors a sample of the output sample received from tap 116.
In the error amplifier chain, the signal from splitter 120 is delayed by delay module 122 (to compensate for the timing of the corresponding portion of the main amplifying chain). At coupler 124, the portion of the amplified signal received from tap 110 is subtracted from the delayed signal from delay module 122 to generate an error signal. Adjuster 130 (optionally) adjusts the amplitude and/or phase of the error signal prior to application to EA 132. The amplified output from EA 132 is the feed-forward error-compensation signal that is added to the delayed, amplified signal from delay module 112 at coupler 114 to generate the output signal. Detector 128 monitors a sample of the error signal received from tap 126 prior to the error signal being applied to adjuster 130.
As indicated in FIG. 1, amplifier circuit 100 has two loops: a nulling loop (i.e., Loop 1 in FIG. 1) and an error loop (i.e., Loop 2 in FIG. 1). According to the prior art, amplifier circuit 100 is tuned by first tuning the nulling loop and then tuning the error loop. In particular, the nulling loop is tuned by applying an input signal to amplifier circuit 100 (with pilot generator 104 turned off) and using nulling-loop adjuster 102 to adjust the amplitude and/or phase of its applied signal until the power of the error signal detected by detector 128 is minimized. After the nulling loop has been tuned and with the input signal typically still present, the error loop is then tuned by (i) injecting a known pilot signal (e.g., one or more continuous wave (CW) signals or a spread-spectrum signal) from pilot generator 104 at coupler 106 and, (ii) with nulling-loop adjuster 102 locked to its tuned setting, using error-loop adjuster 130 to adjust the amplitude and/or phase of the signal in the error amplifier until the power of the pilot signal detected by detector 118 is minimized (e.g., ideally zero).
In order to maintain tuning of a real-world amplifier system in which operating characteristics vary over time with changes in the input signal, the ambient temperature and humidity, and the like, the system-tuning process consisting of first tuning the nulling loop followed by tuning of the error loop is typically continuously or at least periodically repeated to dynamically adjust the operations of amplifier circuit 100.
In order for detector 118 to be able to distinguish the presence of the amplified pilot signal from the amplified input signal, the pilot signal injected at coupler 106 must be different in some way from the input signal. In some prior art implementations, pilot generator 104 is designed to generate the pilot signal as a CW signal having a frequency different from those frequencies contained in the input signal. In this case, detector 118 is typically implemented as a narrow-band detector that is able to detect the presence of the amplified CW pilot signal in the otherwise wide-band output signal.